Most aircraft hydraulic fluids used in civilian aircraft contain some combination of phosphate esters including trialkyl phosphates, dialkyl aryl phosphate esters, alkyl diaryl phosphate esters and tri aryl phosphate esters. Such formulations are disclosed in RE 37,101 to Deetman and are said to provide superior thermal, oxidative and hydrolytic stability. A hydraulic fluid useful in aircraft is available from applicants' assignee under the trademark Skydrol® LD4. This composition typically contains 18 to 25% by weight dibutyl phenyl phosphate, 50 to 60% by weight tributyl phosphate, 4 to 8% of butyl diphenyl phosphate, 5 to 10% of viscosity index improvers, 0.13 to 1% of a diphenyldithioethane copper corrosion inhibitor, 0.005% to about 1% by weight, but preferably 0.0075% to 0.075% of a perfluoroalkylsulfonic acid salt antierosion agent, 4% to 8% by weight of an acid scavenger of the type described in U.S. Pat. No. 3,723,320 and about 1% by weight of 2,6-di-tertiary-butyl-p-cresol as an antioxidant. This composition has proved highly satisfactory in high performance aircraft application.
Since the publication of the Deetman patent, various formulations varying from the base stock disclosed by Deetman have been published to provide hydraulic fluids of varying properties. U.S. Pat. Nos. 6,319,423 and 6,649,080 to Okazaki et al. disclose variations. These base stocks contain a major amount of trialkyl phosphates wherein the alkyl portion is preferably isobutyl or isopentyl and a minor amount of triaryl phosphate. Typical base stock formulations are those containing from 30% to 45% triisobutyl phosphate, 30% to 45% tri-n-butyl phosphate and 10% to 15% triaryl phosphate. Other base stock formulation disclosed in these patents include those having 35% to 45% triisobutyl phosphate, 40% to 50% tri-n-butyl phosphate and 12% to 16% triaryl phosphate. Because of the concern for attack by hydraulic fluid degradation products on the elastomers employed in hydraulic systems seals, the Okazaki et al base stocks provide a mixture of triisobutyl phosphate and tri-n-butyl phosphate together with an amount of triaryl phosphates such that the fluid will produce no more than 25% elastomer seal swell under standard test procedures wherein the amount of triisobutyl phosphate ranges from about 35% to about 50% based on the total weight of the base stock. It is stated that such fluids have a combination of properties useful as aircraft hydraulic fluid compositions including acceptable hydrolytic stability, high flash point, good antiwear properties, acceptable erosion protection, acceptable low temperature flow properties and elastomer compatibility.
While the above noted patents indicate a high degree of effort to provide fluids useful in hydraulic aircraft fluid systems with optimum properties, the aircraft industry continually increases demands for higher requirements. Demand for overall improved properties of the hydraulic fluids is caused by ever higher performance aircraft being flown. Therefore, there is a need for even greater level of performance with regard to fluid life, (thermal stability and low temperature viscosity, while maintaining acceptable fire/flash points, auto ignition temperature as well as compatibility with materials used in aircraft hydraulic systems).